Nuclear fuel rods for nuclear reactors typically comprise a plurality of discrete nuclear fuel pellets disposed within a Zircaloy cladding tube. The pellets are stacked within the cladding tube which is then evacuated, back-filled with helium and sealed by welding Zircaloy end plugs at each of the opposite ends of the tube. Typical Zircaloy cladding tubes may have outside diameters, for example, on the order of 0.4-0.5 inches and lengths on the order of 150-160 inches. The end plugs per se, depending on whether they are upper or lower end plugs in use, may have a length of approximately 31/2 inches or smaller and a diameter substantially approximating the diameter of the fuel rod cladding.
In the manufacture of end plugs, the Zircaloy material from which the end plugs are formed initially comprises a large diameter ingot, e.g., on the order of 36 inches. By various thermal and mechanical operations, the ingot is reduced down from such large diameter to barstock size, for example, barstock approximating between about 0.25 and 0.75 inches. These thermal and mechanical operations include a substantial number of labor-intensive and costly steps such as a series of forging and reforging operations, extrusions and swaging with reheat therebetween. This reduced barstock is normally provided in random, but long, lengths, on the order of many feet. Once the barstock has been formed, machining operations are performed directly on the barstock to finally shape the end plugs. Typically, the barstock is fed through a chuck on a screw machine and machined directly to the final shape of the end plug. Alternatively, small pieces or blanks can be preformed, e.g., by a forging operation, and the final end plugs formed by a chucking machine.
Prior to forming the end plugs from the barstock material, non-destructive tests are typically performed on the barstock in order to detect and remove internal defects which are a result of and inherent to the previously described thermal and mechanical operations necessary to reduce the ingot from its large diameter to barstock size. Internal defects may also arise on occasion as a result of using contaminated materials, including recycle material. These non-destructive tests may include ultrasonic tests, metallographic tests on the barstock ends or liquid penetrant inspection on the ends of the bars. The purpose of the tests is to detect and identify defects which might provide leakage paths for gas or liquids within the fuel rod cladding once the plugs are welded to the cladding and the fuel rods are charged with helium. Tests for detecting helium leakage paths in barstock, however, have not always detected internal defects which would permit leakage through the end plugs. It is believed that leakage paths, to the extent they occur in the end plugs, are an inherent result of the current methods of reducing of the material from the large-diameter ingot to barstock and particularly the thermal and mechanical operations necessary to do so. This large reduction, including when extruding, tends to provide an axial pipe which can be elongated by swaging, i.e., when squeezing the metal down to the smaller size. Thus, axial or centerline defects occur as a result of the manufacture of the barstock and are carried over into the end plugs manufactured from that barstock. Consequently, end-to-end leakage paths occasionally exist in the final end plugs. While the number of defects is very small in comparison with the number of fuel rods manufactured, the problem addressed by the present invention is to further reduce this rather small failure rate, for example, down to a failure rate of less than one end plug per million fuel rods manufactured.
Moreover, in addition to these technical reasons for reducing the failure rate, the reducing operations typically require a large number of thermal and mechanical operations, as previously described. Those operations require substantial time and labor. Hence, the processing becomes time-consuming, expensive and incurs material loss.